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Electrochemistry and Electrical Circuits and Their Elements MCAT-3200184 book November 13, 2015 14:38 MHID: 1-25-958837-8 ISBN: 1-25-958837-2 CHAPTER 3 Electrochemistry and Electrical Circuits and Their Elements Read This Chapter to Learn About ➤ Electrostatics ➤ Electric Circuits ➤ Magnetism ➤ Electrochemistry ➤ Specialized Cells––Nerve Cells ELECTROSTATICS A simple static shock, the beating of the heart, the operation of household appliances, and the devastating damage inflicted by a lightning bolt—all of these examples in nature involve applications of electrostatics. Electrostatics is the study of electrically charged particles—their properties such as mass and charge, their behavior such as conservation of charge, and their interactions such as the repulsive or attractive forces that occur and the calculation of the magnitude of such forces through Coulomb’s law. This chapter reviews the fundamental concepts of electrostatics. Electric Charge and Charge Conservation Electric charge q is a physical property of the basic building blocks of the atom,a fundamental property of all matter. The SI unit of charge is the coulomb, abbrevi- ated C. Although charge can be positive or negative, the magnitude of charge is 53 MCAT-3200184 book November 13, 2015 14:38 MHID: 1-25-958837-8 ISBN: 1-25-958837-2 54 − UNIT I: e = 1.6 × 10 19 C. Considering the particles of the atom, the charge of the positively − Physical charged proton is +1.6 × 10 19 C, and the charge of the negatively charged electron is Foundations of − −1.6 × 10 19 C. Two like charges (either two positive charges or two negative charges) Biological Systems repel each other. Positive and negative charges attract each other. Electric charge is a conserved quantity and thus follows conservation of charge: Electric charge can neither be created nor destroyed, only transferred. The net charge of a system remains constant. Conductors and Insulators As you will read in the following section on electric circuits, it is important to identify materials that will either allow or prevent the flow of electric charge. Conductors such as metals are materials that allow the storage of or facilitate the flow of electric charge. Insulators such as rubber or wood prevent the storage or flow of electric charge. Electric Force: Coulomb’s Law Coulomb’s law describes the electrostatic force Fel between two charged particles q1 and q2, separated by a distance r: q q q q = 1 1 2 = 1 2 Fel k 4πεo r2 r2 −12 2 2 where εo is the permittivity constant, defined as εo = 8.85 × 10 C /N · m . Values for k are: 1 N · m2 k = = 9.0 × 109 4πεo C2 Electrostatic force, as is the case for all types of forces, is a vector quantity and is expressed in units of newtons (N). The direction of the electrostatic force is based on the charges involved. Unlike charges generate an attractive (negative) force, and the direction is toward the other charge; like charges generate a repulsive (positive) force, and the direction is away from the other charge. EXAMPLE: Determine the electrostatic force between two alpha particles of − − charge +2e (3.2 × 10 19 C) separated by 10 13 m. SOLUTION: The electrostatic force can be determined using Coulomb’s law, · 2 . × −19 . × −19 = × 9 N m 3 2 10 C 3 2 10 C Fel 9 10 C2 1 × 10−13 m 2 − = 9.22 × 10 2 N, repulsive MCAT-3200184 book November 13, 2015 14:38 MHID: 1-25-958837-8 ISBN: 1-25-958837-2 55 CHAPTER 3: Electric Field Electrochemistry Electric field E defines the electric force exerted on a positive test charge positioned and Electrical Circuits and Their at any given point in space. A positive test charge, q , is similar in most respects to o Elements a true charge except that it does not exert an electrostatic force on any adjacent or nearby charges. Thus the electric field of a positive test charge provides an idealized distribution of electrostatic force generated by the test charge and is given by: Fel E = qo Because E is a vector quantity, the direction is dependent on the identity of the charge. Because the test charge is positive, if the other charge is negative, an attractive force is generated and the direction of E is toward the negative charge. Likewise, if the other charge is positive, a repulsive force is generated and the direction of E is away from the positive charge. Electric field E is expressed in units of newtons per coulomb. If E is known, it is possible to determine the electrostatic force exerted on any charge q placed at the same position as the test charge using: = Fel qoE An electric field can be produced by one or more electric charges. The electric field of a point charge, which always points away from a positive charge and toward a nega- tive charge, can be calculated by direct substitution of Coulomb’s law into the expres- sion for E: qq k o Fel 2 q E = = r = k qo qo r2 ELECTRIC FIELD LINES Electric field lines represent a visual display of the electric field that uses imaginary lines to represent the magnitude and direction of the electric field or the distribution of the electrostatic force over a region in space. The lines of force from a positive charge are directed away from the positive charge, whereas the lines of force of a negative charge are directed toward the negative charge, as depicted in Figure 3-1. The magni- tude of the force is greater in the region closer to the charge and becomes weaker as the distance from the charge increases. ELECTRIC FIELD DUE TO CHARGE DISTRIBUTION For more than one charge in a defined region of space, the total electric field Etot, because it is a vector quantity, is the vector sum of the electric field generated by each charge Eq in the distribution, or = + + + +··· Etot Eq1 Eq2 Eq3 Eq4 MCAT-3200184 book November 13, 2015 14:38 MHID: 1-25-958837-8 ISBN: 1-25-958837-2 56 UNIT I: Repulsive Electrostatic Force of Like Charges Physical Foundations of Biological Systems + + Attractive Electrostatic Force of Unlike Charges - + FIGURE 3-1 Electric field lines. Source: From George Hademenos, Schaum’s Outline of Physics for Pre-Med, Biology, and Allied Health Students, McGraw-Hill, 1998; reproduced with permission of The McGraw-Hill Companies. Electric Potential The concept of potential in this regard is similar to the potential that was discussed in Chapter 1. It discussed the fact that potential energy becomes stored by an object as a result of work done to raise the object against a gravitational field. The electric poten- tial V at some point B becomes stored as a result of work done, W, against an electric field to move a positive test charge from infinity (point A) to that point (point B), or −W V = qo work Electric potential = charge The electric potential is a scalar quantity that can be positive, negative, or zero, depending on the sign and magnitude of the point charge as well as the work done. Electric potential is expressed in units of joules per coulomb (J/C) = volts (V). The electric potential difference, V, between any two points A and B in an elec- tric field is related to the work done by the electrostatic force to move the charge from point A to point B as: −W = − = AB V VB VA qo MCAT-3200184 book November 13, 2015 14:38 MHID: 1-25-958837-8 ISBN: 1-25-958837-2 57 CHAPTER 3: Electrochemistry and Electrical Circuits and Their Elements FIGURE 3-2 Equipotential surfaces generated for a positive charge. The absolute electric potential at a point A that exists at a distance r from a charged particle at point B depends on the magnitude of charge at point B as well as the distance according to the following formula: q V = k r This is only true if one assumes that V → 0 at a point infinitely far away. Because the electric potential is a scalar quantity, the electric potential for n charges can be deter- mined by adding the electric potential values calculated for each of the n charges: n = = + + + +···+ V Vi Vq1 Vq2 Vq3 Vq4 Vqn i=1 Equipotential surfaces are a graphical method of representing the electric poten- tial of any charge distribution as concentric circles that are normal or perpendicular to electric field lines. Consider the example of a positive charge in Figure 3-2. The electric field vectors are pointed away radially in all directions from the charge. Equipoten- tial surfaces can also be drawn on the diagram to represent the electric potential of a positive charge at any distance r from the charge. Recall that the electric potential of a q point charge is given by V = k . On the surface of the charge where r is at its minimum, r the electric potential is at its greatest, and thus a solid circle is drawn about the point charge to represent the largest magnitude of the potential at the surface of the point charge. As the distance r increases, the electric potential is represented as concentric circles that become larger in circumference. EXAMPLE: The Bohr model of the hydrogen atom describes electron motion in a − circular orbit of radius 0.53 Å (angstrom, where 1 angstrom = 1 × 10 10 m) about the nuclear proton. Determine the following for the orbiting electron: 1. The electric field 2. The electric potential MCAT-3200184 book November 13, 2015 14:38 MHID: 1-25-958837-8 ISBN: 1-25-958837-2 58 UNIT I: SOLUTION: Physical Foundations of 1.
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